The present invention relates to a light beam scanning apparatus for simultaneously scanning and exposing a single photosensitive drum with a plurality of laser beams, thereby to form a single electrostatic latent image on the photosensitive drum, and an image forming apparatus such as a digital copying machine or a laser printer using the light beam scanning apparatus.
In recent years, various digital copying machines have been developed in which image formation is performed by scanning and exposing with a laser beam and electronic photographing processing.
More recently, in order to obtain higher image forming speed, developments have been made to a digital copying machine adopting a multi-beam method in which a plurality of laser beams are generated and scanning is simultaneously carried out for a plurality of scanning lines with use of a plurality of beams.
This kind of digital copying machine which adopts such a multi-beam method comprises a plurality of laser oscillators for generating laser beams, a multi-face rotation mirror such as a polygon mirror for reflecting the laser beams generated by the plurality of laser oscillator toward a photosensitive drum to scan the photosensitive drum with the laser beams, and an optical unit serving as a light beam scanning device consisting mainly of a collimator lens and an f-.theta. lens.
However, in the structure of a conventional optical unit, it is very difficult to obtain an ideal positional relationship between a plurality of light beams on a photosensitive drum (or a surface to be scanned). In order to obtain an ideal positional relationship, respective components as well as assembling thereof require high accuracy, and hence the cost of the device is increased.
Even if an ideal positional relationship is obtained, the shape of a lens may vary slightly or the positional relationship between respective components may vary slightly due to circumferential changes, such as changes in temperature and humidity or time-based changes. Consequently, the positional relationship between light beams varies, and as a result a high quality image cannot be formed. Therefore, to construct this kind of optical system, it is necessary to adopt a structure and components which are strong against changes as described above. In particular, as for lenses, a glass lens which is strong against circumferential changes and time-based changes is expensive so that the cost of the device is increased.
In the following, defects in the multi-beam method, which are caused when an image is formed with light beams whose passing positions are erroneously dislocated, will be explained with reference to FIGS. 32A and 32B and FIGS. 33A and 33B.
For example, in a case where character "T" shown in FIG. 32A is formed, an image as shown in FIG. 32B is formed when a passing position of a light beam is erroneously dislocated from a predetermined position. In the example of this figure, the passing position of a light beam b is shifted from its predetermined position so that the distance between light beam a and b is reduced while the distance between light beam b and c is increased, among four light beams a and d used. In other words, the dot interval changes in the sub-scanning direction.
FIG. 33A shows an example of an image in which emission timings of respective light beams are not controlled correctly. As is apparent from this figure, the image forming position in the main scanning direction is dislocated so that a longitudinal line cannot be formed straight. That is, the dots are shifted from each other in the main scanning direction.
FIG. 33B shows an image in which neither the passing positions of light beams not the emission timings are controlled correctly, defects in an image appear both in the sub-scanning direction and in the main scanning direction.
Thus, when an image is formed in a multi-beam method, light beam passing positions in the sub-scanning direction must be controlled so as to be arranged at predetermined intervals, and the emission timings of respective light beams must be controlled so as to align the image forming position in the main scanning direction.